The random phase diffuser relates to imaging systems in general, and ultrasonic imaging in particular.
In reflecting imaging systems generally, the reflected waves of an image must return to the observing aperture [be it a camera or the human eye]. In optical systems there generally is enough roughness on the surface of the illuminating light source to scatter the light over a broad angular cone. The surface variations are comparable to the wave length of light and, thus, scatter the light rather effectively. Therefore, reflected waves in an optical system can be collected even if the observed surface is not perpendicular to the impinging light.
Unfortunately however, surfaces to be observed in accoustical imaging are quite smooth in terms of the wave length of sound used [around 1 mm]. These surfaces act like mirrors such that the angle of incidence of a sound wave is equal to the angle of reflection. If the surface is not insonified [illuminated accoustically] perpendicular to the collecting aperture, the reflected waves will not return to the aperture. Therefore, to successfully image by reflection, diffuse insonification must be used. Generally, the insonification pattern is used that the subject surface appears to be covered with point reflectors, each of which is radiating into a large angle. In the event the surface is not perpendicular to the insonification radiation, waves from these point reflectors will still return to the aperture as long as the angle of the surface to the radiation is less than the angle through which these points are radiating. With such diffuse insonification, surfaces of complex shape and orientation can be reflectively imaged.
Unfortunately, when diffuse insonification is used with a coherent insonification source, interference is produced. The interference fringes between the point reflectors produce what is known as "speckle," a series of black lines produced by the interference fringes, which overlie the reflected image. One prior art method of eliminating "speckle" from the image is by superposition of a number of uncorrelated patterns [analogous to incoherent light]. The variation caused by the overlaying interference fringes cancel the fringes out and produce a speckleless image. Unfortunately, this requires the handling of numerous patterns, and requires a separate operation for each image produced.